ORIGIN OF THE ROCKS AND SCENERY OF NORTH WALES. 241 



daposited in very deep water, it is necessary to assume depressions during 

 the formation. Towards the close of this great period a remarkable 

 development of igneous action took place. Huge volcanoes were pro- 

 duced somewhere in the district, and great masses of fragmental rocks 

 and crystalline lavas were formed. 



Professor Kamsay divides the igneous rocks of Snowdon into three 

 groups ; — 



1. Columnar felspathic trap, 200ft. 



2. Ashy beds, 1,200ft. 



3. Basement, slaggy and brecciated lavas, 1,700ft. 



This lowest series, north of the Pass of Llanberis, is divided into 

 three groups separated by grits and slates. The volcanic material must 

 have been derived from the interior of the earth, and accordingly we 

 find numerous dykes, veins and necks of igneous rock, traces of the ducts 

 by which communication was established between the exterior and the 

 interior. 



"With this volcanic episode our account of the geological history 

 of North Wales, as far as regards the formation of rocks, must cease. 

 Some time after this, how long we cannot say, the whole mass of 

 rock formed during the Cambrian period must have been subjected to 

 lateral pressure, acting from the N.W. and S.E., and the folds 

 and contortions now seen in the strata of North Wales, together with the 

 slaty cleavage, owe their origin to these disturbances. These folds affect 

 both the igneous and sedimentary rocks alike, and cannot therefore be 

 connected with the volcanic action which produced the former. 

 (To be continued.) 



BKITISH LICHENS : HOW TO STUDY THEM. 



BY W. PHILLIPS, F.L.S. 



(Continued from page 199.) 



In the home study of Lichens, not the least important subject we 

 have to dwell upon is the application of chemical tests for the deter- 

 mining of species. 



That certain Lichens contained colouring matter which could be 

 made available for dyeing cloth is a fact that has been made known for 

 many years, indeed some authors have gone so far as to say there are 

 allusions to the blue and purple colours procured from them for this 

 purpose in the Bible, (Ezekiel xxvii., v. 7,) but it is only within the last 

 twenty years that Lichenologists have sought to turn this character of 

 Lichens into a means for distinguishing species. A solution of iodine 

 was found, when applied to the hymenium of certain species, to yield a 

 blue reaction, while in other species it merely yielded a vinous brown 

 colour. The blue colour is attributed to the presence of starch in minute 

 quantity in the gelatinous matter pervading the hymenium, called by 

 Nylander the gelatina hymenia ; or if the colour be produced in the 

 membrane of the aacus and the cell walls of the paraphyses it is 



