12 BASIC AND ULTRABASIC IGNEOUS ROCKS— BENSON. I MEM0IRS [ ^.Tix! 



gravitationally stratified laccolite of norite with gabbro, banded troctolite, and "picrite," but 

 Read ('19) points out that there are no gradual passages from one rock to another, and the indi- 

 vidual rock-types are well marked. "It seems more probable that the differentiation of the 

 gabbroic magma took place in a lower chamber, and that the picrite was intruded as a small sill 

 followed by increasingly acid derivatives, each of which slid roughly on the top of its fore- 

 runner. After the intrusion of the composite picrite-norite set had finished, there were intruded 

 a few small bosses of diorite, followed by monzonites, granites, and pegmatites." (Op. cit.) 



The date of intrusion of all these masses is rather vague. The inclusion of pebbles of 

 granite in the lower Old Red Sandstone, and the invasion of this sandstone by the granites of 

 Glencoe would suggest a long, continuous period of plutonic activity with recrudescence of 

 intrusion through the interval between late Silurian and Middle Devonian times. 4 



The close of the Highland-folding, and the deposition with continued subsidence in the 

 lowlands of Scotland of Devonian and Carboniferous sediments was accompanied by intru- 

 sions which, commencing in Lower Carboniferous times, reached their maximum in Upper 

 Carboniferous and Permian times. The rocks developed are dolerites, essexites, theralites, 

 teschenites, and true picrites, with basalt and limburgite. They occur especially near Edin- 

 burgh and Glasgow (Bailey and others '10). The remarkable sills of picrite of Inchholm 

 (Campbell andStenhouse '07) and Lugar Water 5 (Tyrrell '09, '12, '17) and the analcite-rocks 

 discussed by Tyrrell (op. cit.) and Scott ('16) are the most noteworthy members of this group 

 of rocks, which are clearly alkaline in character, and very analogous to the Bohemian proto- 

 types of Becke's "Atlantische Sippe." They clearly fall into the alkaline plateau group of 

 eruptions in our classification and owing to the abundance of magmatic water, have been greatly 

 differentiated, the subsidence of the heavier crystals being facilitated by the abnormal fluidity 

 of the magma. 



While, however, the early Devonian conditions ot crust folding and igneous intrusion of 

 the orogenic type were waning in Scotland and passing into the Carboniferous conditions of 

 plateau-subsidence and eruptions, geosynclinal conditions of sedimentation with typical spilitic 

 eruptions had recommenced in the southwest of England, and extended from the Devonian 

 into Carboniferous times. The extension of this geosyncline, the Hercynian trough, passed 

 through Germany, where somewhat similar conditions occurred. In England, the shape of 

 the igneous masses formed at this time has been more or less obscured by later folding and 

 crushing, but the presence of tuffs and pillow-lava, often reduced to the condition of schistose 

 greenstone, intercalated among phyllites and radiolarian sediments, of clearly intrusive albitised 

 dolerites, minverites, palaeo-picrites, and keratophyres, typify the features of the spilitic suite 

 of which this area bas been taken as the type by Flett and Dewey ('11). The subsequent fold- 

 ing and overthrusting in later Carboniferous times, of the geosyncline in which these rocks 

 were developed, is the most marked feature (Dewey '09), and this overthrusting was followed 

 by the intrusion of the Hercynian granites of Devon and Cornwall. 6 



It was perhaps during the latter part of the Carboniferous period that the Whin Sill was 

 formed, a sheet of dolerite which extended probably continuously over an area of more than a 

 thousand square miles. It is on the average about a hundred feet in thickness, but diminishes 

 toward the west; rarely 'it splits into two portions. It is not quite concordant, but lies with a 

 very slight obliquity to the bedding planes of the inclosing Carboniferous limestone, as in 

 different districts it appears in different horizons in the Carboniferous succession. No trace 

 of any boss or neck which might be supposed to mark a funnel of ascent for the material of the 

 Whin Sill has been detected (Geikie '97, Harker '17). The intrusion of Carrock Fell (Fig. 2) 

 may also be of this age, though possibly it should be referred to the Tertiary period (Harker, 

 '94, '09, '17). It is interesting as being the example chosen by Harker to illustrate dif- 

 ferentiation by crystallization with active diffusion in the magma. Bowen ('15) has urged 



* There is a long dike of serpentine believed to invade the Old Red Sandstone near Kinnordy in Forfarshire. No modern data concerning it are 

 yet available. 



fl The writer was guided over this region by Mr. Tyrrell. 



• The writer is indebted to Mr. Dewey for guidance through this region . 



