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yorkshire petrology. by thomas tate, f.g s. 
Introduction. 
The Igneous Rocks of Yorkshire are restricted within an area 
bordering on the north-west and northern boundaries of the county. 
Volcanic rocks, both contemporaneous and intrusive, have here 
representatives whose ages may be fairly well determined. If, as 
Clifton Ward has suggested, the volcanic series of the Lake District 
had a sub-aerial origin, at the same time as the Landeilo Flags were 
accumulating in the Welsh seas, this "marks time" for the con- 
temporaneous Volcanic Rocks extending from Ribblesdale to Teesdale, 
as well as for the Borrowdale Group of the Lake District, of which 
indeed these form a part, the easterly and south-easterly fringe. 
The Erupted Rocks of Yorkshire belong to two younger, but 
widely separated epochs. A varied series of trap-dykes cuts through 
the contemporaneous Volcanic Rocks just named, all along the line ; 
behaving transgressively also towards the remaining Ordovician and 
and Silurian Groups wherever present, but never intruding into beds 
of Carboniferous age. In one instance where we find the unaltered 
scar-limestone wrapping unconformably around the denuded apex 
of a protruding dyke, the area of contact presents some points of 
interest, as we hope to show. Clearly, these dykes are of Old Red- 
sandstone age, and, like the Volcanic Rocks of the Borrowdale 
series, were erupted during a period of elevation. 
The youngest Intrusive Rocks, the great Cleveland Dyke, 
stretching along the northern limit of the county, we shall see reason 
for assigning in like manner to a period of elevation, in all likelihood 
of Middle Eocene age. 
Mica-trap, like felstone, trachyte or basalt, is a term convenient 
for the grouping of rocks in field work, to which indeed its vague 
character lends itself ; hence the nomenclature of the groups has, 
until recently, been wanting in precision. Rocks of widely different 
mineral constitution have been included under this head, indeed it is 
no uncommon experience to find specimens in museums, or slides for 
