234 Transactions of tJie [Sess. 



VIII.—iiKETGH OF THE GEOLOGY OF THE PENTLANDS. 



By Mr JOHN HENDERSON. 



{Read Feb. 26, 1885.) 



The Pentland Hills, lying so near the city, have always been a 

 favourite resort of Edinburgh naturalists ; and no doubt most 

 members of the Edinburgh Naturalists' Field Club are familiar with 

 the zoology and botany of these hills. But as I understand the 

 members of the Club have not as yet devoted so much atten- 

 tion to another kindred branch of natural science — viz., Greology — 

 I thought it might not be out of place to give a brief sketch of the 

 geology of the Pentland Hills, hoping to direct the attention of 

 some of the members, at any rate, to this by no means the least 

 interesting of the physical sciences. 



The Pentland Hills geographically occupy a belt of country 

 extending from the south side of Edinburgh to Dunsyre in Lanark- 

 shire. They run north-east and south-west, and are about 16 miles 

 in length and from 3 to 6 in width, and their greatest height above 

 the sea-level is about 1900 feet. In the north-east portion of the 

 range, where the hills attain their greatest elevation, the principal 

 rocks are of volcanic origin, while in the middle and south-west 

 portions the rocks are mostly of sedimentary character. Before 

 going into detail regarding these rocks, I take the liberty of giving 

 you a list of the different sedimentary formations or systems that 

 form the known crust of the earth, so as to keep us in mind of the 

 position the rocks forming the Pentland Hills occupy in the geolo- 

 gical record. The different systems or formations are arranged in 

 the following order, beginning at the lowest — viz.: Archaean, 

 Cambrian, Silurian, Devonian or Old Red Sandstone, Carbonifer- 

 ous, Permian, Triassic, Jurassic, Cretaceous, Tertiary, and Post- 

 Tertiary. Such is the arrangement of what is known as the 

 sedimentary deposits — that is, rocks formed by the deposition of 

 mud, sand, gravel, boulders, and other materials, through the agency 

 of water ; while the term volcanic or igneous is applied to all those 

 rocks which have been ejected from below, either in the condition 

 of lava, ashes, or volcanic mud, or consolidated between the strata 

 into which they have been injected. The term contemporaneous 

 is applied to those volcanic products which have reached the 

 surface and become interbedded in the sedimentary deposits ; 

 while the term intrusive is applied to those that have been con- 

 solidated between the rocks into which they have been injected. 



The north-eastern portion of the Pentland Hills is composed 

 principally of igneous or volcanic rocks, consisting of a series of 



