The Cockburn Law intrusion is a small sheeted boss of late Caledonian age, intruded into Silurian rocks of Llandovery age.

The following are the chief stages in its formation:—

1. The earliest igneous activity was the intrusion of basic rock, now only found as xenoliths. This intrusion was followed at some later stage by—

2. The main plutonic magma which assimilated the basic rock and country rock to produce the following hybrids—

(a) Hybrid hypersthene diorite, from contamination by the Silurian pelites and greywackes.

(b) Hybrid diopside diorite, from contamination by the basic rock.

(c) Hybrid pyroxene diorite, from contamination by material from both basic igneous and sedimentary rocks.

3. After the consolidation of the marginal hybrids the main body of the magma began to solidify, but had time to differentiate in place to give a composition ranging from adamellite through granite to granophyric microgranite.

4. The heat from the intrusion metamorphosed the Silurian sediments to biotite cordierite hornfels which grade insensibly into the hybrid hypersthene diorite.

5. Following the consolidation of the plutonic mass there was considerable activity by vapours to form pneumatolytic products, while differentiation of the residual magma produced the aplitic, acidic, and lamprophyric dykes.

The intrusion may form a cupola from a bathylith, which may possibly belong to a series of bathyliths lying in a Caledonian direction from Newry to the Firth of Forth.

A series of hypersthene-bearing rocks of plutonic aspect from an area east of Mount Kenya is described, with the assistance of modal analyses. A petrographic classification of such rocks, somewhat more detailed than is usually employed, is suggested, with provision for hornblende-rich types. Associated rocks are briefly described.

In 1914 the British Museum (Natural History) acquired from William McPherson, the well-known collector, a few ostracoderms from the “Downtonian” of Lanarkshire. Among these were two specimens from Logan Water of an obviously new form, which, however, were so preserved that satisfactory identification by the methods then in use was not possible. They were tentatively labelled “allied to Lasanius” and are the “interesting though tantalizing new specimens” referred to by Smith Woodward (1921, p. 26) in his first Linnean Address. Nothing further was done with them for nearly twenty years, when the rearrangement of the collections again drew attention to these problematical fossils: and subsequent investigation has shown them to be of considerable importance.