lxxii GENERAL SUMMARY OF SCIENTIFIC AND 



Islands, both those of the Carboniferous and those of the Ter- 

 tiary age, and finds the older and the later precisely alike in 

 constitution and in structure, though the older he conceives 

 to have undergone subsequent changes. The mineralogical 

 elements of these dolerites include, besides feldspar, augite, 

 and chrysolite, magnetite, apatite, mica, and a glassy matrix. 



E. S. Dana has just given us the preliminary results of an 

 important microscopic study of the trap rocks of the Connecti- 

 cut Valley, the harder and more crystalline varieties of which, 

 according to him, consist of labradorite, pyroxene, and mag- 

 netite, more rarely with chrysolite and apatite. The pyrox- 

 ene in the coarse-grained dolerite often appears inlong-bladed 

 prisms somewhat resembling hornblende, and hitherto mis- 

 taken for that mineral. Some varieties of this rock contain 

 four or five per cent, of water, and hold much chlorite, part- 

 ly in plates, seeming as if formed from the pyroxene, and 

 partly in cavities. The soft, light-green amygdaloidal vari- 

 eties are highly hydrated, and contain chlorite, quartz, and 

 datolite. These he conceives may result from a subsequent 

 change of the anhydrous dolerites. 



Hull and Judd have described the great volcanic outflows 

 seen in the north of Ireland and the Hebrides. According to 

 the former, there were no traces of volcanic rocks in the 

 Lower Cambrian, but in the Upper Cambrian are found sheets 

 of acidic feldspar rocks, with others hornblendic. These he 

 regards as probably resulting from submarine eruptions. 

 This was also a period of volcanic activity in Wales and in 

 Cumberland. In the Upper or true Silurian period sheets 

 of quartziferous porphyry are found alternating with sand- 

 stones, conglomerates, and shales. Similar phenomena occur 

 in the Devonian sandstones in Ireland, where in Killarney is 

 a mass of columnar feldstone, an old volcanic neck or throat, 

 near to which are beds of volcanic ash and conglomerate, in- 

 cluding hollow balls or volcanic bombs. Similar volcanic 

 necks are seen in the Carboniferous, and outflows of lavas, 

 sometimes acidic, but chiefly basic, consisting of dolerites, 

 containing, besides a triclinic feldspar and augite, chrysolite, 

 chlorite, and magnetite. In Tertiary times there was appar- 

 ently a still more abundant outflow of lavas in this region, 

 w T hich formed a part of the great volcanic belt to which Ice- 

 land, Central France, the Iberian Peninsula, the Azores, Ma- 



