THE 



ALGA-FLORA OF YORKSHIRE. 



Yorkshire, being much the largest county in England, is in 

 consequence of its diversified topographical characters fairly 

 rich in freshwater algse, although as compared with either West- 

 morland or Cumberland it lacks many of the more uncommon 

 species. This is due to the diflferent character of the geological 

 formations, which consist of carboniferous, oolitic, and other 

 limestones, shales, grits, and alluvial deposits. The mountain- 

 ous portion of the county is not so productive as such a region 

 would be were its strata composed of the older Palaeozoic rocks, 

 or if it possessed some rocks of an eruptive, igneous origin. In 

 the extreme north-west a trap-dyke does occur in a very limited 

 area, and this appears to have considerably augmented the rich- 

 ness of that district. In addition to the Silurian rocks near 

 Sedbergh, there are a few other such small inliers of this forma- 

 tion, but they do not appear to have much influence on the 

 general alga-flora. The mountainous region, known as the 

 north-western fells, is a district of lofty hills, thirty-six of which 

 attain an altitude of 2,000 feet or more, the highest point being 

 Mickle Fell (2,596 feet). There are many sub-alpine algse 

 found in this extensive district, and many rare ones, but only 

 a few of the upland bogs are productive of the rarer species, 

 most of which are obtained from small springs high up on the 

 mountains and from the dripping rocks of the ghylls and glens. 

 In this district the three largest natural sheets of water are 

 Malham Tarn (altitude 1,250 feet) Semer Water, and Birkdale 

 Tarn, the first-mentioned being much the most productive. 



Bot. 5— A 



