September 22, 1911] 



SCIUNGE 



363 



whole but not all parts equally, and with 

 this we must connect those groups of ig- 

 neous rocks which have a wide distribution 

 throughout the province. But there have 

 also been movements in the lateral sense, 

 more strictly localized and more sharply 

 accentuated, and to these belong evidently 

 the plutonic rocks with various other 

 groups which are their satellites. I have 

 pointed out these facts elsewhere, but 

 failed to follow out the logical conclusions 

 on the petrographical side. Influenced by 

 the strongly marked characters of the plu- 

 tonic series, I assigned the North British 

 Tertiary rocks, not without some misgiv- 

 ings, to the calcic or Pacific region. Suess, 

 having regard probably to the broader 

 tectonic features rather than to petro- 

 graphical data, has included our area in 

 the Atlantic region. 



Concerning the calcic facies of the plu- 

 tonic rocks there can be no question. They 

 constitute a well-defined "rock-series," in- 

 truded in order of decreasing basicity, and 

 ranging from ultrabasie to thoroughly acid. 

 The ultrabasie rocks, as developed in Rum 

 and Skye, have a lime-felspar as one of 

 their chief components: there are no pic- 

 rites (in the original sense of Tschermak) 

 or other alkaline types. The eucrite group, 

 found in Rum, Ardnamurchan and the 

 Carlingford district, is also characterized 

 by a felspar near anorthite. Gabbros are 

 represented at nearly all the several cen- 

 ters, and in Arran they are accompanied 

 by norites. The granites and granophyres 

 fall into two sub-groups. The less acid is 

 usually augitic, while the more acid, found 

 in Arran, St. Kilda and the Mourne Moun- 

 tains, carries hornblende and sometimes 

 biotite. 



This series is kno^vn in various prov- 

 inces of Pacific facies. A peculiarity of it 

 is that it is a broken series, types of mean 

 acidity being absent. This has an interest- 



ing consequence. In many places a granite 

 magma, invading rocks so different from 

 itself as gabbro or eucrite, has caused 

 energetic mutual reactions, and a set of hy- 

 brid rocks has been produced, which serves 

 in a limited sense to fill the gap in the 

 series. 



The only known exceptions to the calcic 

 facies of our Tertiary plutonic rocks are 

 perhaps significant in that they occur near 

 the northern and southern limits of the 

 principal belt of activity. The massive 

 gently inclined sheets of granite and gran- 

 ophyre which make up part of the south- 

 ern end of Raasay consist largely of 

 microperthite, and contain abundant rie- 

 beckite, a distinctively alkaline mineral 

 known at only one spot in Skye. The 

 microperthitic granites of Arran do not 

 carry riebeckite, but it is found in the well- 

 known rock of Ailsa Craig, farther south. 



The local groups of minor intrusions — 

 acid, basic and ultrabasie — related to the 

 several plutonic centers have the same 

 calcic facies as the plutonic rocks of which 

 they are satellites. It appears, however, 

 that they sometimes tend to a more alka- 

 line composition towards the borders of 

 their respective districts. Thus, the Skye 

 granite is surrounded by a roughly oval 

 area, within which are found numerous 

 dykes and sills of felsite and granophyre, 

 in general augitic ; but on the fringe of the 

 area these rocks give place to orthophyres, 

 with biotite or hornblende, and to boston- 

 ites. 



Turn now to the rocks of regional distri- 

 bution. The most important are, of 

 course, the basalt lavas. They are all fel- 

 spar-basalts, but a very general feature is 

 the filling of their numerous amygdaloidal 

 cavities with zeolites, such as analcime, 

 natrolite, chabazite and stilbite. These 

 minerals are certainly not mere weather- 

 ing-products. When I examined the ba- 



